The present invention relates to a numerically controlled machine tool such as a roll grinder for grinding a workpiece into a roll shape and a method of controlling grinding operation of the numerically controlled machine tool, and more particularly to a numerically controlled machine tool capable of machining a workpiece to a desired shape while correcting wear, damage, and the like of a workpiece and taking into account elastic deformation of the workpiece, wear of a tool, and the like.
Numerically controlled machine tools, typically roll grinders for grinding workpieces into roll shapes, are generally arranged to grind the workpiece into a roll shape while rotating the workpiece at a prescribed speed, moving a tool radially toward and away from the workpiece and axially of the workpiece. Since the tool has to be moved, the machine tool has a memory for storing numerical control data by which amounts of radial movement of the tool are successively programmed dependent on axial positions of the tool, and a control unit for supplying command pulses to drive units to move the tool based on the numerical control data. The numerically controlled machine tools are widely used for machining rolls or the like.
Rolls to be machined by numerically controlled machine tools include differently shaped rolls ranging from simple cylindrical rolls to rolls having taper, sine, and arcuate outer peripheral shapes to meet various applications. Heretofore, rolls having such outer peripheral shapes have generally been ground by employing cams. Use of cams is however disadvantageous in that a suitable cam has to be selected when a roll to be ground has a different outer peripheral shape, a different length, a different radius, or other different parameters, and that the efficiency of the grinding process is low.
It is possible to machine a workpiece relatively simply into a desired roll which has an outer peripheral shape that can be represented by a relatively simple formula, such as a sine curve. In this machining process, a workpiece portion to be machined, is divided into small segments, each of which is approximated by a straight line or an arc to produce numerical control data (machining program), and the produced numerical data are used to control the machine tool. However, since machining programs thus produced are very long and complex, they are not suitable for manufacturing rolls having complicated outer peripheral shapes.
For grinding a workpiece highly precisely into a roll, the depth of cut by which the workpiece is radially ground by a tool (i.e., a rotatable grinding wheel) is of great importance during the grinding process. If there were no other factors or elements to be taken into consideration, then a target (command) depth of cut would be the same as a net depth of cut, making it possible to grind the workpiece accurately.
On a roll grinder, however, the rotatable grinding wheel is worn as the grinding process goes on, and hence the actual depth of cut is smaller than a given target depth of cut. In addition, since the grinding wheel is pressed against the workpiece under certain pressure during the grinding process, the workpiece or roll is slightly elastically deformed. The amount by which the roll is elastically deformed is also responsible for reducing the actual depth of cut smaller than the given target depth of cut.
High-precision grinding of a workpiece into a roll therefore requires a machining program to be prepared while taken into account wear on the grinding wheel, and elastic deformation of the roll. Inasmuch as the amounts of correction to be effected by the machining program for wear and elastic deformation vary from grinding wheel to grinding wheel and from roll to roll, predictive values for the amounts of correction have to be determined with respect to each grinding wheel and each roll. The amount of wear of the grinding wheel varies with time in the grinding process, and it is highly difficult to prepare the numerical control program taking such a time-dependent variation in the amount of wear into consideration. One practice for grinding a workpiece using a machining program prepared on predictive values has been to measure the dimensions of a roll to confirm the dimensional accuracy thereof when the grinding wheel approaches a target cutting position, correct the depth of cut, and start the grinding process again. This grinding operation is however very poor in efficiency.
Rolls which have been in use for certain periods of time are worn differently dependent on what they have been used for and how they have been used. Even a single roll has a portion which is worn to a larger extent and a portion which is worn to a smaller extent. For efficiently grinding such a roll, therefore, the portion which is less worn has to be intensively ground to a desired roll shape, a process known as a truing process.
In the truing process, an individual roll shape is measured to check how it is worn, and based on the measurement, a machining program is prepared for grinding the roll. Alternatively, a machining program for grinding the roll to a uniform diameter irrespective of how it is worn is used to grind the roll. According to the former procedure, machining programs have to be prepared respectively for rolls to be ground, dependent on the worn conditions of the rolls. The latter uniform grinding method does not require machining programs to be prepared for the respective rolls, but results in a long machining time since the rolls must be ground uniformy regardless of their worn conditions.